Electrochemical batteries are generally used to provide direct current and power in a large variety of different operations. Batteries utilizing the reactivity of lithium metal have been known. It has been, however, observed that the operation of a battery incorporating lithium metal in elemental form may become hazardous under certain circumstances. Further research in this field lead to the development of lithium ion batteries in which elemental lithium is replaced by substances intercalating lithium ions. Such intercalating substances are capable of absorbing substantial amounts of lithium ions and reversibly releasing the lithium ions in a subsequent operation.
A conventional lithium ion battery has a negative electrode comprising an active material which releases lithium ions when discharging and intercalates or absorbs lithium ions when the battery is being charged. The positive electrode of a lithium ion battery comprises an active material of a different nature, one that is capable of reacting with lithium ions on discharge, and releasing lithium ions on charging the battery. In some of the conventional lithium ion batteries the negative electrode is separated from the positive electrode by a perforated or microporous membrane or continuous layer, made of some suitable organic polymer. The external faces of the electrodes are usually equipped with some means to collect the charge generated by the battery during discharge, and to permit connection to an external power source during the recharging of the lithium ion battery. The current collectors are usually made of stainless steel, iron-nickel alloys, copper foil, aluminum and similar relatively inexpensive metals. The conventional lithium ion battery also comprises a lithium ion containing electrolyte, which may be either a non-aqueous liquid or a solid organic polymer, the lithium ion therein being supplied by dissociation of a lithium salt dissolved in the electrolyte. An exemplary lithium ion battery is described in U.S. Pat. No. 5,187,033, issued to N. Koshiba on Feb. 17, 1993.
As referred to above, when the level of performance of a lithium ion battery falls below that desired the battery may be recharged. The useful life of a rechargeable or of a secondary battery is determined by the number of times it may be recharged without noticeable deterioration in its performance. It is known that ionic movement in the proximity of the current collector of a battery may cause corrosion of the current collector. More particularly, corrosion of the current collector in contact with the electrodes of a lithium ion battery may be the result of one or more of the following phenomena: the highly reactive nature of lithium ions, high potentials encountered during recharging of a lithium battery, relatively low corrosion resistance of the metals utilized as current collectors in lithium ion batteries and events of similar nature. It is to be noted that the current collector working in conjunction with the positive electrode is more prone to corrosion, however the current collector in contact with the negative electrode may also be corroded. A corroded current collector may lead to uneven battery power delivery, or even to complete breakdown in the performance of the battery. It is therefore of great importance that corrosion of the current collector is minimized in the charge and discharge operations of a lithium ion battery in order to ensure a long and useful battery life.
U.S. Pat. No. 5,187,033 utilizes in one of its embodiments fine powder of non-corrodible conductive metals mixed with the active material to diminish corrosion of the current collector. The non-corrodible metal in the lithium ion battery of U.S. Pat. No. 5,187,033 is silver or platinum, which may be used in conjunction with fine carbon also incorporated with the active material. In another embodiment of U.S. Pat. No. 5,187,033 silver or platinum is plated on the current collector facing the negative electrode. The plating may be replaced by a net of platinum and silver. It is assumed that unless the silver or platinum layer is of measurable thickness, this type of corrosion protection is likely to break down early in the life of the lithium ion battery. Silver or platinum of measurable thickness may substantially increase the cost of production of lithium ion batteries.
U.S. Pat. No. 5,262,254 issued to Koksbang et al. on Nov. 16, 1993, describes an electrically conductive organic polymer layer placed between the positive electrode and the metallic current collector of a lithium ion battery. Koksbang et al. list several organic compounds which may be utilized in obtaining an electrically conductive organic polymer corrosion protective layer inserted within a lithium ion battery. In another embodiment of Koksbang et al. both sides of the metallic current collector in the proximity of the electrode are enclosed in an electrically conductive organic polymer film or layer. It is considered that the cost of production of lithium ion batteries may be substantially increased by incorporating relatively expensive electrically conductive organic polymers therein. Moreover, the conductivity of such organic polymers is usually less than that of conventionally used metallic current collectors.
There is a need for a relatively inexpensive method to diminish, and preferably eliminate corrosive interaction between the active material and the current collector in lithium ion batteries.